KVM: guest_memfd: abstract how prepared folios are recorded

Right now, large folios are not supported in guest_memfd, and therefore the order
used by kvm_gmem_populate() is always 0.  In this scenario, using the up-to-date
bit to track prepared-ness is nice and easy because we have one bit available
per page.

In the future, however, we might have large pages that are partially populated;
for example, in the case of SEV-SNP, if a large page has both shared and private
areas inside, it is necessary to populate it at a granularity that is smaller
than that of the guest_memfd's backing store.  In that case we will have
to track preparedness at a 4K level, probably as a bitmap.

In preparation for that, do not use explicitly folio_test_uptodate() and
folio_mark_uptodate().  Return the state of the page directly from
__kvm_gmem_get_pfn(), so that it is expected to apply to 2^N pages
with N=*max_order.  The function to mark a range as prepared for now
takes just a folio, but is expected to take also an index and order
(or something like that) when large pages are introduced.

Thanks to Michael Roth for pointing out the issue with large pages.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

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